The present invention relates to a nitride semiconductor laser device.
A semiconductor laser device using a nitride semiconductor such as GaN, InN or AlN can generate light in the green to blue regions and is expected to be a light source for a high-density optical disk apparatus. A nitride semiconductor laser device of the type emitting light in the blue part of the spectrum will be described as an exemplary prior art device.
FIG. 11 illustrates a conventional nitride semiconductor laser device 600. In the device 600, n-type GaN electrode forming layer 62 (including upper and lower parts 62a and 62b) n-type GaAlN cladding layer 63, InGaN/GaN multi-quantum well (MQW) active layer 64, p-type GaAlN cladding layer 65 and p-type GaN electrode forming layer 66 are formed in this order on a sapphire substrate 61. An electrode provided on the n-type side (n-type electrode) 67, made up of multiple pairs of Ti/Al layers alternately stacked, is formed on the lower part 62a of the n-type electrode forming layer 62. On the other hand, an electrode provided on the p-type side (p-type electrode) 68, made up of multiple pairs of Ni/Au layers alternately stacked, is formed on the p-type electrode forming layer 66. In this manner, a laser diode 60, also called a "laser element or cavity" is formed. On both facets of the laser diode 60, from/by which laser light is emitted or reflected, a pair of SiO.sub.2 or SiN protective layers 69 are provided, thus preventing the deterioration of the laser facets. In this case, SiO.sub.2 or SiN need not have their compositions exactly defined by stoichiometry. Instead, these layers 69 should have resistivity (or insulating properties) and refractive index that are substantially equal to those of SiO.sub.2 or SiN. In this specification, part of a semiconductor laser device, from which stimulated emission of radiation is produced, will be referred to as a "semiconductor laser diode", and a combination of the semiconductor laser diode with at least one protective or reflective layer a "semiconductor laser device" for convenience.
The conventional nitride semiconductor laser device 600 may be fabricated by the following method. First, the electrode forming layer 62, cladding layer 63, MQW active layer 64, cladding layer 65 and electrode forming layer 66 are formed in this order by a crystal-growing technique on the sapphire substrate 61. Thereafter, respective portions of the electrode forming layer 66, cladding layer 65, MQW active layer 64, cladding layer 63 and upper part 62b of the electrode forming layer 62 are etched, thereby exposing the upper surface of the lower part 62a of the electrode forming layer 62. The n- and p-type electrodes 67 and 68 are formed on the exposed upper surface of the electrode forming layer 62a and the electrode forming layer 66, respectively, by an evaporation technique. Thereafter, the pair of protective layers 69 are formed on both laser facets by a sputtering or electron beam (EB) evaporation technique.
FIGS. 12A and 12B illustrate another conventional nitride semiconductor laser device 700. The device 700 includes: n-type GaAlN cladding layer 72; InGaN/GaN MQW active layer 74; p-type GaAlN cladding layer 75; and p-type GaN electrode forming layer 76, which are stacked in this order on a sapphire substrate 72 by a crystal-growing technique. An Ni/Au electrode 77 and a Ti/Al electrode 71 are formed on the upper and lower surfaces of this multilayer structure to form a laser diode 70. In order to reduce the operating current of this laser diode 70, a reflective layer 90, made up of four pairs of SiO.sub.2 /TiO.sub.2 layers 91, 92 alternately stacked with the thickness of each layer defined as .lambda./4n (n is a refractive index of each layer 91 or 92), is formed on the rear facet, or the back, of the laser diode 70. On the front facet, or the front, of the laser diode 70, an SiO.sub.2 protective layer 80 is formed at a thickness defined as .lambda./2n (n is a refractive index of the protective layer 80). Herein, .lambda. is an oscillation wavelength of the laser diode 70. The stimulated emission of radiation is output from the front. The front protective layer 80 and the rear reflective layer 90 are deposited by a sputtering or EB evaporation technique.
By providing the reflective layer 90 on the back, the reflectance of the back increases to about 98%, and almost all laser light can be emitted from the front. As a result, the operating current can be reduced to about 70% of that consumed by a semiconductor laser device with only an SiO.sub.2 protective layer formed on its back at an ordinary thickness defined by .lambda./2n.
However, the lifetime of the conventional nitride semi-conductor laser devices 600 and 700 are short particularly when operating at high output power. The present inventors found that the lifetime of these nitride semiconductor laser devices are short because of the following reasons:
(1) the laser diodes 60 and 70 are made up of a plurality of crystal layers, whereas the protective layers 69 and 80 and the reflective layer 90, formed on the facets thereof are formed of SiO.sub.2 or TiO.sub.2 and are all amorphous layers. In addition, the length of a bond in the material for these amorphous layers (e.g., the length of an Si--O bond) is different from the lattice constant of the crystal layers in the laser diodes. Accordingly, lattice mismatching is caused in these interfaces to create lattice defects in these crystal layers (in the MQW active layer, in particular). Moreover, if the protective layers 69 and 80 and the reflective layer 90 are formed on the laser facets by a sputtering or EB evaporation technique, then these laser facets would be damaged due to relatively high impact energy of material particles flying from the target. As a result, lattice defects might be caused in the crystal layers in the laser diodes 60 and 70.
(2) The thermal expansion coefficients of the crystal layers in the laser diodes 60 and 70 are greatly different from those of the protective layers 69 and 80 and the reflective layer 90. Accordingly, the crystal layers (the MQW active layer, in particular) are strained while the protective layers 69 and 80 and the reflective layer 90 are cooled down to room temperature after these layers have been formed and during the operation of the devices (during high-power operation, in particular). As a result, crystal defects are newly created or the number thereof increases. For example, the thermal expansion coefficient of the MQW active layer 64 is 3.15.times.10.sup.-6 k.sup.-1, which is greatly different from that of the protective layer 69 at 1.6.times.10.sup.-7 k.sup.-1.